This invention relates to a vehicle air conditioning system including a variable displacement refrigerant compressor, and more particularly to a method of controlling an electrically activated compressor displacement control valve.
Variable displacement refrigerant compressor have been utilized in automotive air conditioning systems to reduce accessory load fluctuations associated with on/off cycling of a fixed displacement compressor. In a typical arrangement, a variable displacement compressor includes one or more pistons coupled to a wobble plate, and the tilt angle of the wobble plate is adjustable to vary the stroke of the pistons, and hence the displacement or pumping capacity of the compressor. Adjustment of the wobble plate angle is achieved by changing a differential pressure acting on a wobble plate control mechanism, and the differential pressure is controlled by a displacement control valve that varies the pressure in a crankcase of the compressor. While the control valve may be implemented pneumatically, an electrically activated configuration (such as a pulse-width-modulated or linear control valve) lends itself to the implementation of electronic control algorithms. For example, a microprocessor-based controller can activate an electrical control valve to regulate compressor displacement as required to maintain the evaporator outlet air temperature at a desired value, taking into account other system parameters such as high-side or low-side refrigerant pressure. See, for example, the U.S. Pat. No. 6,092,380 to Kachur et al., issued on Jul. 25, 2000, and assigned to the assignee of the present invention. However, nonlinearities and delays in the control response of the compressor and the air conditioning system in general sometimes make it difficult to achieve a stable and reliable control without adding undue complexity to the control algorithm. Accordingly, what is needed is an easily implemented control algorithm that accommodates the non-linear characteristics of the compressor and air conditioning system for regulating compressor displacement to achieve a stable and reliable system control.
The present invention is directed to a non-linear fuzzy logic control method for an air conditioning system including an electronic controller and a variable displacement compressor, wherein the controller carries out the control method to electrically activate a compressor displacement control valve for regulating the compressor displacement as required to maintain a system parameter, such as the evaporator outlet pressure or temperature, at a desired value. The control inputs include the system parameter to be controlled and a refrigerant pressure that reflects changes in the compressor displacement. The output of the fuzzy logic control is a requested change in control valve activation, and the activation of the control valve is updated based on the requested change.